This invention relates to a new and improved steel particularly adapted for vitreous enameling applications and process for producing such steel. More specifically, this invention relates to a vanadium-nitrogen bearing rimmed steel, and process for producing the same, which steel is particularly adapted for one-coat porcelain enameling.
Steels produced for use in enameling applications may be classified in two categories: one-coat and two-coat. Two-coat enameling steels generally contain above .01% carbon, with some of the carbon being in the form of iron carbide with the base matrix. When such steels are coated with a porcelain frit and fired, surface carbides react with the frit to produce carbonaceous gas, which in turn produces pits or gas bubbles in the enameled surface. An additional coat is then applied to cover these defects and to produce a smooth finished surface. One-coat enameling steels are generally decarburized to a carbon level below 0.007%. Consequently, no free carbides exist in the base matrix. Since, under such condition, there is little or no danger of carbide reaction with the enameling frit to form defects, such materials may require only a single coat of frit to produce the desired surface. It is apparent that such one-coat steels result in considerably saving of time and money and why they have come to be in great favor among enamelers.
However, the decarburized one-coat steels that have been available, heretofore, usually suffer from severe loss of strength after the forming and firing operations. Such loss is usually produced by the thermal and mechanical treatments to which the material has been subjected during processing. During forming, cold work is introduced into the material, with strain levels ranging up to 20%. The subsequent firing of the frit coating to the base metal results in recrystallization and grain growth, and a concomitant loss of strength. Flexing or other work loading of the enameled part may then produce yielding of the base metal. The brittle enamel surface does not yield and hence spalls from the base metal. The spalled or chipped material then must be either scrapped or re-enameled. Consequently, such materials are restricted to those applications where higher-fired strengths are required.
The desideratums for direct one-coat enameling steels include: resistance to formation of gas producing defects; good pickling rate; adequate formability; suitable resistance to sag and distortion at enameling temperatures; and adequate strength after enameling. The one-coat steels which have been available heretofore to the porcelain enameling industry do not exhibit all of these desirable characteristics to a suitable degree; for example some of the titanium stabilized steels exhibit a good yield strength after firing characteristic but do not present a suitable surface appearance.